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Labour hit out at Boris Johnson, London’s mayor, on Sunday after he floated the idea of a new airport in the Thames estuary – suggesting it could be a device to detract from the Conservative party’s divisions over the expansion of Heathrow.

David Cameron, the Tory leader, has started to sound more lukewarm over Heathrow’s proposed extra runway, but pro-business voices within his party want the project to go ahead.

The government has already considered the proposal for an airport off the Isle of Sheppey, but concluded that such a project would be hugely costly and problematic. Some estimates have put the cost as high as £40bn ($73bn), including associated transport links and facilities.

An airport in the Thames would cause environmental problems and there would be a threat to aircraft from high local bird density. “This is a Tory attempt to take the spotlight off their own divisions over Heathrow,” said one Labour official.

Mr Johnson, who first endorsed the idea during this year’s mayoral election, believes that a new airport would help solve congestion at Heathrow, which serves about 67m passengers a year. The proposal follows the chaotic opening of Terminal 5 this year when thousands of passengers were stranded as staff struggled to cope with the new systems.

On Sunday, the mayor’s office said he was keen to know what the alternatives were to expansion at Heathrow. A detailed feasibility study would now be undertaken.

Proposals for an airport in the Thames have been around for decades. Under the mayor’s proposal, the new airport would have four runways and operate 24 hours a day. It would be connected to the high-speed Channel tunnel rail link in order to transport passengers into central London in just over half an hour.

Mayoral advisers recently outlined some of their plans to Sir Richard Branson’s Virgin Atlantic airline. But on Sunday, the company appeared lukewarm. “In the short term, London has to have a third runway to keep its competitiveness,” it said.

“In the longer term, it is important to have a debate and we await the findings of the studies with interest, but it is likely the enormous costs associated with this would rule out the building of such an airport.

“The mayor’s job is to protect London’s competitive position, which is why he should be supporting a third runway at Heathrow. Only then should he focus on the long-term needs.”

Business leaders argue that another Heathrow runway is vital to keep London competitive. Earlier this month, leading businesses and trade unions took out a series of press advertisements backing this view.

“It offers the direct connections which make our companies globally successful and which will be all the more important as India and China grow,” the advertisements said. “That’s been recognised by our European competitors – Paris, Amsterdam and Frankfurt airports will each have at least four runways by 2012.”